>> But this is a dead letter as we have seen because there is no way
>> that users can make private agreements with major systems software
>> providers.
>
> Correct. But I think you may have overblown expectations about
> what one should be able to do with PUA code points and what
> major systems software providers should be able or be required
> to support for them.

In theory, I would think one should be able to do whatever the heck one
wants with PUA code points, so long as it involves "characters" in the
Unicode sense of the term, used in some way for representing text.

What major software providers can be expected to support is another
matter entirely. Very little commercial software supports most of the
Unicode character properties at all, other than relatively basic
concepts like uppercase and lowercase. Show me a general-purpose,
commercial software product that understands numeric properties and
interprets them, or groups characters together that have the same
General Category, or uses canonical combining classes flexibly to
perform normalization. If there are any, there aren't many, and if we
can't get that kind of support for assigned characters, it's not likely
we'll see it for the PUA.